My 2 cents...
The downside of leveling/mastering a single weapon at a time is that our Notoriety inflates with the single weapon, diminishing the points earned from the other weapons, actually making them harder to level instead of easier as you suggest. Each weapon mastered would compound the difficulty of the next. This is why most prefer to level up thier weapons somewhat evenly, at least for the first 10-15 levels.
Leveling that first 1 or 2 weapons by doing just them will make mastering THEM much easier.. but the weapons following are going to be a bit more difficult.
I would have to recommend leveling your weapons somewhat evenly, its also gets a bit monotonous using one weapon the entire time
